A relief valve is known in which a servomotor has the shape of an annular ram with an annular cylinder located on the circumference of a discharge tube and has an annular piston which is axially movable by means of a fluid medium different from that flowing through the flow passage constituted by the discharge pipe. This piston is fixed to a cylindrical valve element which is axially displaceable toward and away from a conical water-flow deflector lying on the axis of the pipe so as to open and close the annular outlet defined in part by this conical deflector. The conical deflector itself is placed at the junction between a narrow inlet pipe and a wide outlet pipe and is intended to deflect the inflowing water outwardly so as to dissipate the kinetic energy of the discharge water. During closing the cylindrical valve element of this relief valve moves in the direction of flow of the water passing through the valve and when the annular outlet is partially opened, the valve body protrudes from the smaller pipe into the larger pipe so as to obstruct the flow of water in a stream having an annular cross section at this point.
This arrangement has several disadvantages. First of all in a partially open condition of the valve the cylindrical valve element protrudes from the smaller pipe into the larger pipe and obstructs the flow in the cup-shaped water stream by creating a so-called shadow between the outer surface of the cylindrical valve element and the contour of the transitional area between the smaller pipe and the larger pipe. This creates vortices and underpressures that produce vibration. To suppress underpressures in this shadow space air must be supplied. Furthermore because of the above-mentioned shadow in the flow of discharge water between the outlet side of the cylindrical outwardly protruding closing element and the contour of transition of the smaller pipe into the larger pipe, it is not possible to increase the diameter of the larger pipe beyond a certain degree because the shadow in the water flow becomes too large and increases underpressure, eddying, and vibration. Double-sided guiding of the cylindrical closing element in the annular cylinder of the servomotor has the consequence that water-pressure caused deformation, which is higher with large-diameter pipes, attains a very high value interfering with sliding of the closure element.
It has been suggested to avoid this by limiting stress and providing an exceptionally stiff and, consequently, heavy structure at a considerable increase of cost of the valve and transportation and mounting thereof. The known relief valve also has various technical difficulties. For instance a precise machining of sealing and guiding surfaces, which is essential, is an extremely tedious and expensive job due to the large sealing and guiding surfaces. In order to obtain good sliding the large sliding surfaces must be made of various high-quality and expensive materials.